I don't think Tether will ever reach $0, it will never fall too deep and it will never rise too high. That's why Tether is called a stablecoin. maybe some things that might happen in the future will happen like a war or something and it will definitely have an effect because when that happens, the world is not fine.
Even a war won't decrease the value of Tether to something zero. It might even use more.
Hard regulations might come and it might affect Tether's price but Tether is currently the most used stable coin. From a trader's perspective, it's good to have a stable coin for any crypto trading pair to minimize their losses or secure profits without worrying much about price volatility.
I wouldn't trust Tether with my money even if it offers attractive interest rates. As it's said in the real world, it's "better to be safe than sorry". You're better off exchanging your crypto to Fiat than a centralized stablecoin than Tether. Just my thoughts 
In the first place, why consider Tether as an investment? Just use the advantage of it. It's not that you will put the majority of your baskets on Tether.